The invention relates to a method and a device for optical shape recording and/or evaluation of objects and surfaces, in particular glossy surfaces. The term glossy refers below to objects whose optical roughness lies in the transition range between optically rough and optically smooth surfaces. Optically rough surfaces are defined as having a roughness which is substantially greater than the wavelength of visible light (about 0.5 micrometers), while optically smooth surfaces have a roughness much less than the wavelength. Because of this property, optically rough surfaces exhibit nondirectional, diffuse reflection or transmission of light. Examples of this are paper, chalk, matt disks etc. Optically smooth surfaces, however, reflect or transmit incident light directionally. They are capable of producing an optical image of their surroundings. Examples which may be mentioned are flat or curved mirrors and polished metal and glass surfaces (lenses).
In the transition range between these two extremes lie the objects referred to as glossy. These objects are of great importance since they are encountered very often. In particular, industrially produced objects of metal, plastic or even wood and other materials belong to glossy objects. The industrial processing of such materials (machining of metal and wood, injection molding of plastic, powder injection of metal and ceramic etc.) produces roughnesses in the range of a few micrometers, i.e. of the order of the wavelength of visible light (around 0.5 micrometers).
There is a wide selection of optical 3D sensors for diffusely scattering, optically rough surfaces. One of the most widespread methods is based on the projection of strip patterns. The patterns are projected in one direction and observed with a camera in another direction. Depending on the shape of the object being observed, the strips appear more or less deformed to the camera. The shape of the object can be inferred from the deformation of the strips. More than three strip patterns are generally projected, with the intensity of the strips assuming a sinusoidal profile.
Among the many other methods, the methods of the “shape from shading” group should be mentioned, in particular the photometric stereo method since the invention is based on it. From the brightness structure of an object surface, these methods infer its shape. A detailed description will be given below.
Methods which allow three-dimensional measurement for smooth surfaces are also known. Primarily interferometric methods are employed for testing simple surface shapes, such as flat or spherical surfaces (lenses, mirrors etc.). The Hartmann method or the Shack-Hartmann test are employed for more complexly shaped surfaces such as aspheres. Here, the deflection of a thin beam of rays by the object to be measured is observed. Other methods observe a grid pattern which is reflected or transmitted by the object surface. Depending on the shape of the latter, the grid appears more or less deformed. These methods can be combined under the heading deflectometric methods. A feature common to them is that the ray deflection is determined and the shape of the surface is inferred therefrom. The deflectometric methods are based on the reflection law or refraction law, which describes the relation between an incident ray, surface normal and the reflected or transmitted ray.
The measurement of surfaces in the transition range between optically rough and optically smooth surfaces, however, has not yet been resolved. The methods of both categories are deficient in this case. Although a sensor for rough surfaces can cope with occasionally occurring glossy points, such a sensor is unsuitable when gloss dominates over diffuse scattering. On the other hand, a sensor for optically smooth surfaces, in particular a deflectometric sensor, will have difficulty when the surface is too rough to allow clear optical imaging. For example, it is necessary to ensure that the fine structure of the grid is still visible. The method with sine strips places less stringent requirements on the quality of the surface, since sinusoidal strips allow a greater degree of haziness. But even here it is necessary to ensure that the structure of the strips is still visible.
The known optical sensors thus do not provide satisfactory results precisely for glossy surfaces in the transition range, which occur very frequently in industrially manufactured products.
It is therefore an object of the invention to provide a method and a device which avoid this disadvantage.